Conventionally, a thermal type flow control system MFC and a pressure type flow control system FCS are widely used in a gas supplying apparatus for a semiconductor control device, and in recent years, a thermal type flow control system with improved supply pressure fluctuation resistance characteristics is increasingly used.
FIG. 33 shows the arrangement of a pressure type flow control system FCS. This pressure type flow control system FCS includes a control valve CV, a temperature detector T, a pressure detector P, an orifice OL, and an arithmetic and control unit CD, etc. The arithmetic and control unit CD includes a temperature correction/flow rate arithmetic circuit CDa, a comparison circuit CDb, an input-output circuit CDc and an output circuit CDd, etc., and has excellent characteristics keeping stable flow control characteristics against fluctuation of a primary side supply pressure.
Referring to FIG. 33, detection values from the pressure detector P and the temperature detector T are converted into digital signals and input into the temperature correction/flow rate arithmetic circuit CDa, and here, temperature correction of the detected pressure and flow rate computation are performed, and then the computed flow rate value Qt is input into the comparison circuit CDb. In addition, an input signal Qs of a set flow rate is input from a terminal In, and converted into a digital value by the input-output circuit CDc, and then input into the comparison circuit CDb, and, thereafter, compared with the computed flow rate value Qt from the temperature correction/flow rate arithmetic circuit CDa. As a result of the comparison, when the computed flow rate value Qt is larger than the set flow rate input signal Qs, a control signal Pd is output to the drive unit of the control valve CV. Accordingly, the control valve CV is driven in a closing direction, and is driven in the valve closing direction until the difference (Qs−Qt) between the set flow rate input signal Qs and the computed flow rate value Qt becomes zero.
The pressure type flow control system FCS has excellent characteristics in which when the relationship of P1/P2≧approximately 2 (herein referred to as the critical expansion condition) is kept between the downstream side pressure P2 of the orifice OL (that is, the pressure P2 on the process chamber side) and the upstream side pressure P1 of the orifice OL (that is, the pressure P1 on the outlet side of the control valve CV), the flow rate Q of the gas G0 distributed through the orifice OL is Q=KP1 (here, K is a constant), the flow rate Q can be controlled with high accuracy by controlling the pressure P1, and even when the pressure of the gas G0 on the upstream side of the control valve CV greatly changes, the controlled flow rate value hardly changes.
The pressure type flow control system FCS and the thermal type flow control system with pressure fluctuation resistance characteristics are known, therefore, detailed descriptions thereof are omitted here.
However, for example, in the pressure type flow control system FCS, an orifice OL with a minute hole diameter is used, so that the hole diameter of the orifice OL changes over time due to corrosion caused by a halogen-based gas and precipitation of a reactant gas, etc. As a result, the controlled flow rate value of the pressure type flow control system FCS and the actual flow rate value of the gas G0 actually distributed become different from each other, and to detect this difference, so-called flow monitoring has to be frequently performed, and this greatly affects the operability of the semiconductor manufacturing equipment and the quality of a manufactured semiconductor.
Therefore, in the field of pressure type flow control systems, conventionally, a method is widely used for preventing the controlled flow rate value of the pressure type flow control system FCS and the actual flow rate value of the gas G0 actually distributed from becoming different from each other by detecting a change in hole diameter of the orifice OL as early as possible, and, in order to detect the change in hole diameter of the orifice OL, a gas flow rate measuring method using a so-called build-up system or build-down system is adopted in many cases.
However, in the conventional gas flow rate measurement using a so-called build-up system or build-down system, actual gas supply has to be temporarily stopped, and as a result, the operation rate of the semiconductor manufacturing equipment is lowered, or the quality, etc., of a manufactured semiconductor is greatly affected.
Therefore, in recent years, in the field of thermal type flow control systems and pressure type flow control systems, a flow control system with flow monitoring that can easily monitor in real time whether supply gas flow control is being properly performed without temporarily stopping actual gas supply has been developed.
For example, FIG. 34 shows an example. A flow control system 20 with flow monitoring, being a thermal type mass flow control system (mass flow controller), includes a flow passage 23, a first pressure sensor 27a for an upstream side pressure, a control valve 24, a thermal type mass flow sensor 25 provided on the downstream side of the control valve 24, a second pressure sensor 27b provided on the downstream side of the thermal type mass flow sensor 25, a throttle unit (sonic nozzle) 26 provided on the downstream side of the second pressure sensor 27b, an arithmetic and control unit 28a, and an input-output circuit 28b, etc.
The thermal type mass flow sensor 25 includes a rectifier body 25a inserted into the flow passage 23, a branched flow passage 25b branched by a flow rate of a predetermined proportion of F/A from the flow passage 23, and a sensor main body 25c provided on the branched flow passage 25b, and outputs a flow rate signal Sf showing a total flow rate F.
The throttle unit 26 is a sonic nozzle that provides a fluid at a flow rate corresponding to a primary side pressure when a pressure difference between the primary side and the secondary side of the throttle unit is greater than or equal to a predetermined value. In FIG. 34, the reference symbols SPa and SPb denote pressure signals, Pa and Pb denote pressures, F denotes a total flow rate, Sf denotes a flow rate signal, and Cp denotes a valve opening degree control signal.
The arithmetic and control unit 28a feed-back controls the control valve 24 by feeding-back pressure signals Spa and Spb from the pressure sensors 27a and 27b and a flow rate control signal Sf from the flow sensor 25 and outputting a valve opening degree control signal Cp. That is, a flow rate setting signal Fs is input into the arithmetic and control unit 28a via the input-output circuit 28b, and the flow rate F of the fluid flowing in the mass flow control system 20 is adjusted so as to match the flow rate setting signal Fs.
In detail, by controlling the opening and closing of the control valve 24 by feed-back controlling the control valve 24 by the arithmetic and control unit 28a by using the output (pressure signal Spb) of the second pressure sensor 27b, the flow rate F of the fluid flowing in the sonic nozzle 26 is controlled, and by using an output (flow rate signal Sf) of the thermal type flow sensor 25 at this time, the flow rate F of the actual flow is measured, and, by comparing the measured value of this flow rate F and the controlled value of the flow rate F, the operation of the mass flow control system 20 is confirmed.
Thus, in the flow control system 20 with flow monitoring shown in FIG. 34, two measuring systems of pressure type flow rate measurement using the second pressure sensor 27b for performing flow control and flow rate measurement using the thermal type flow sensor 25 for monitoring the flow rate are installed in the arithmetic and control unit 28a, so that, whether or not the fluid at the controlled flow rate (set flow rate Fs) is actually flowing, that is, whether or not the controlled flow rate and the actual flow rate are different from each other can be easily and reliably monitored in real time, so that a high practical effect is obtained.
However, many problems that should be solved still remain in the flow control system 20 with flow monitoring shown in FIG. 34.
A first problem is that since two different flow rate measuring systems of pressure type flow rate measurement using the second pressure sensor 27b for performing flow control and flow rate measurement using the thermal type flow sensor 25 for monitoring the flow rate are utilized, the structure of the flow control system 20 with flow monitoring becomes complicated, and the system cannot be downsized and reduced in manufacturing cost.
A second problem is that the arithmetic and control unit 28a is arranged to control the opening and closing of the control valve 24 by using the signals of both of an output SPb of the second pressure sensor 27b and a flow rate output Sf of the thermal type flow sensor 25, and correct the flow rate output Sf of the thermal type flow sensor 25 by using an output SPa of the first pressure sensor 27a, and opening and closing of the control valve 24 are controlled by using three signals of the two pressure signals SPa and SPb of the first pressure sensor 27a and the second pressure sensor 27b and the flow rate signal Sf from the thermal type flow sensor 25. Therefore, not only does the make up of the arithmetic and control unit 28 become complicated, but also the stable flow control characteristics and excellent high responsiveness of the pressure type flow control system FCS are lessened adversely.